1. Field of the Invention
The invention concerns a method and a device for controlling the temperature of hot tap-water, which is heated on the secondary side of a heat exchanger, wherein the temperature and the flow of the hot tap-water is measured and wherein a regulator controls a setting means governing the flow at the primary side of the heat exchanger.
2. Description of Related Art
Such a device is previously known from the German published patent application DE 4206074 A1, comprising, on the primary side of the heat exchanger, a closed circulating circuit including a storage container for hot water and a circulation pump. The circulation pump is controlled by a regulator in response to the temperature drop over the heat exchanger at the primary side as well as the temperature drop over the heat exchanger at the secondary side, the regulator being programmed to calculate the amount of energy which the heat exchanger must transfer per time unit. Here, another parameter being taken into account, is the flow of hot tap-water which is sensed by a flow sensor in the secondary circuit. For such a calculation, a plurality (at least four) of temperature sensors are thus required as well as associated connections to the regulator. Even so, with this known device, it is difficult to accomplish a good temperature stability in case of strongly varying consumption of hot tap-water.
Similar arrangements with heat exchangers are being used also in district central heating networks, wherein the forward and return lines of the network are connected to the primary side of the heat exchanger along with a setting means for governing the flow of municipal hot water through the heat exchanger.
In housing or the like, whether there are only a few housing units or a great number of units, the consumption of hot tap water varies strongly and relatively quickly during certain time periods each day. When the flow changes, the temperature sensor will sense such a change only upon a certain delay, when the volume of water in the secondary circuit has flown away and been replaced by fresh water which has not yet been heated to the desired set temperature. Consequently, there will be a considerable drop of the temperature of the hot tap-water before the regulator has a possibility to react and control the flow regulating valve in the primary circuit of the heat exchanger.
Accordingly, a problem which occurs when heating hot tap-water in a heat exchanger is to keep the water temperature constant when the flow thereof varies strongly. An attempt to solve this problem is described in the international patent application published in WO 86/06459 (Cairenius), where the secondary circuit is connected to a mixer, in which hot water, which has passed the heat exchanger and has been heated therein, is mixed with cold water (water which has not been heated), the mixing relation being controlled by means of a thermostat valve. Hereby, large flow variations can be permitted, but the dynamic response will nevertheless be insufficient. The measure of letting the thermostat control the setting means of the primary circuit will in principle cause the same problem as discussed above, which is due to the fact that the state of a thermostat is directly dependent on the temperature of the water flowing in the secondary circuit.
In still another known device, disclosed in the Swedish patent specification SE-B-328 388 (Overgaard), the consumer water is heated in a heat exchanger, the primary side of which is connected to a district central heating network. Here, one has chosen to dispense with the thermostat valve or a corresponding temperature sensor in the consumption water circuit (the hot tap-water). Instead, there is a flow sensor which directly controls a valve in the line connected to the central heating network. A fluctuation in the flow of hot tap-water will cause a quick response, but the absolute temperature of the hot tap water is not controlled at all and may vary with different parameters, such as the temperature of the incoming consumer water, the temperature of the water in the central heating network, etc.